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Regioselective Reactivity of an Asymmetric Tetravalent Di[dihydroxotin(IV)] Bis‐Porphyrin Host Driven by Hydrogen‐Bond Templation
Author(s) -
Brotherhood Peter R.,
Luck Ian J.,
Blake Iain M.,
Jensen Paul,
Turner Peter,
Crossley Maxwell J.
Publication year - 2008
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200801775
Subject(s) - steric effects , porphyrin , chemistry , ligand (biochemistry) , regioselectivity , denticity , hydrogen bond , stereochemistry , carboxylic acid , supramolecular chemistry , reactivity (psychology) , electronic effect , crystallography , molecule , photochemistry , crystal structure , organic chemistry , catalysis , receptor , medicine , biochemistry , alternative medicine , pathology
Local molecular environment effects on the rates of ligand exchange at an asymmetric di[dihydroxotin(IV)] bis‐porphyrin 5 are examined. The host 5 possesses four non‐equivalent tin(IV)–ligand binding sites that are distinguished by their position relative to a shallow cavity, by the steric environment at each binding site and by electronic‐structure differences between the constituent porphyrin and quinoxalinoporphyrin macrocycles. These design features of the asymmetric host are confirmed by X‐ray crystal structure analysis. Binding experiments with monodentate carboxylic acids and bidentate dicarboxylic acids show significant differences in the rate of ligand exchange at each of the four tin(IV) binding sites. For monodentate carboxylic acids, binding preferentially occurs at the exterior porphyrin site. Further addition of carboxylic acid results in sequential binding at the quinoxalinoporphyrin sites and lastly at the interior site on the porphyrin, with high regioselectivity. These selective binding outcomes are immediately apparent by NMR spectroscopy. A series of 2D NMR spectroscopy experiments allowed identification of the preferred binding sites at the host. This positively identifies that steric hindrance and electron‐withdrawing functionality on the porphyrin macrocycle impede ligand exchange. However, these effects are overcome by dicarboxylic acid guests, which form ditopic hydrogen‐bond interactions between the intracavity hydroxo ligands in the initial stage of ligand exchange, leading to regioselective binding between the tin(IV) sites within the cavity. It is envisaged that the factors identified herein that define regioselective ligand exchange at host 5 will find wider application in supramolecular systems incorporating tin(IV) porphyrins.